An M code known as maximum length shift register sequences is a linear code which can be produced by a significantly simple circuit arrangement, and hence is used widely in spread spectrum communication systems. FIG. 5 is a block diagram of a prior art simple-type M code generator including flip-flops SR.sub.1 through SR.sub.6, an exclusive 0R gate .sym., an input terminal IN receiving a clock pulse CL and an output terminal OUT outputting an M code responsive to the incoming clock pulse CL in a known fashion.
Due to the linearity, M codes are inferior to nonlinear codes from a standpoint of signal hiding. Therefore, M codes have not been regarded as important when a high privacy is required.